Various technologies are available for manufacturing grinding rods for use in grinding mills, such as in ore crushing, stone crushing and the like. Grinding rods are usually 3 to 6 meters in length depending upon the size of the grinding device and have diameters which usually range from 7 to 10 cm. It has been found that the useful life of a grinding rod may be improved if it has a hard outer shell usually of martensitic microstructure and relatively soft end portions which are substantially of pearlitic microstructure. The soft end portions minimize rod spalling and splitting thereof and reduce breakage and wear of the rod mill liners. A discussion of grinding rods having soft end portions may be found in U.S. Pat. No. 4,589,934 as well as the several other US patents discussed in the background of that US patent.
In an attempt to improve grinding rod longevity by way of heat treatment, the chemistry of the steel in the grinding rod may be modified such as described in U.S. Pat. No. 4,840,686. The modification of the chemistry in the steel of the grinding rod results in the rod core having a bainitic microstructure with less than 10% pearlite and a core hardness of at least about 40 Rockwell C, or 40 HRC. It is thought that making rods with the proper selection of molybdenum and chromium to provide the bainite core enhances the wear rate of the rod by nearly 20% over that of a conventional heat treated rod.
The rods, as made in accordance with either of U.S. Pat. Nos. 4,589,934 and 4,840,686 are quenched after heating by passing the rod through a quench spray. The quenching of the rod is commenced inwardly of the leading end of the rod and the quench spray turned off short of the trailing end of the rod. It is thought that by not applying quench water spray to the leading end and trailing end of the rod, softer end portions are developed. Also as taught, the rod may have to pass through multiple quench zones in order to achieve the desired extent of quenching to ensure the formation of the harder martensitic shell. As is described in U.S. Pat. No. 4,589,934, minor amounts of quench water travelling along the rod surface towards either the leading or trailing end portion may create a wash effect, thereby expediting cooling of the end portion resulting in the formation of end portions which can have a hardness greater than 30 and perhaps up to 45 or 50 HRC. To minimize this effect, the commencing of the quench water spray and terminating of the quench water spray are activated or deactivated a considerable distance from each end. A significant portion of the rod end is not treated resulting in a fairly large transition zone between the quench portion of the rod which has the martensitic structure and the untreated end portion of the rod which has the pearlitic structure. In practice, the softer end portions of the rod may extend upwards of 30 cm or more with a very gradual transition from the hard shell to the softer portion. This results in a grinding rod having a greater length of softer end portion with consequent increased wear.
In accordance with an aspect of this invention, a grinding rod is provided which overcomes the above problem in providing an engineered heat treated end portion of less than 15 cm and having a hardness of less than 35 Rockwell C.